1. Field of the Invention
The invention relates to a method for processing an OFDM (Orthogonal Frequency Division Multiplex)-signal.
2. Description of Related Art
The OFDM technique is an efficient transmission technique to cope with many channel impairments. By inserting a guard interval between OFDM symbols, inter symbol interference (ISI) can be mitigated. Nevertheless, a robust time and frequency synchronization is necessary to achieve good performance of an OFDM system in severe channels. For many OFDM systems the transmission is organized in OFDM frames, which consist of several OFDM symbols. Therefore, a robust OFDM frame synchronization method is necessary, too.
A fractional time offset can be determined by calculating the correlation of the guard interval or parts thereof with the corresponding parts at the end of an OFDM symbol. With the same method an estimate of a fractional frequency offset, can be calculated. After the determination of the fractional time offset and the fractional frequency offset, the FFT window is positioned and a FFT (Fast Fourier Transformation) is done. The result of the FFT are received cells (cf. below). However, after this pre-processing method, a coarse frequency offset in multiples of the sub-carrier spacing and the beginning of a transmission frame, i. e. an OFDM frame, needs to be determined. The coarse frequency offset in multiples of the sub-carrier spacing can also be referred to as integer frequency offset and the term integer time offset can be used in order to describe the time offset in time units of an OFDM symbol.
After the integer frequency offset and the integer time offset have been determined, a frequency offset compensation and time offset compensation of the OFDM signal is performed.
The error rate of the determination of the integer frequency offset and the integer time offset has a strong influence on the performance of an OFDM receiver, i. e. the reliability of the synchronization of an OFDM receiver.